1，图片左中右三宫格缩放形式
//左右14像素不变形，中间缩放
let imgTrackRight = UIImage(named:"slider_max")
let imgRight = imgTrackRight!.stretchableImageWithLeftCapWidth(14, topCapHeight:0)
slider.setMaximumTrackImage(imgRight, forState:UIControlState.Normal)
2，图片上中下三宫格缩放形式
//上下10像素不变形，中间缩放
let imgBgSouce = UIImage(named:"bg")
let imgBg = imgBgSouce!.stretchableImageWithLeftCapWidth(0, topCapHeight:10)
3，图片9宫格缩放
//左右14像素,上下10像素不变形，中间缩放
let imgBgSouce = UIImage(named:"bg")
let imgBg = imgBgSouce!.stretchableImageWithLeftCapWidth(14, topCapHeight:10)